instrumentation and control of distillation towers
TRANSCRIPT
-
7/28/2019 Instrumentation and Control of Distillation Towers
1/20
INDUSTRIAL PROCESS
CONTROL:
CONTROL OFDISTILLATION TOWERS
-
7/28/2019 Instrumentation and Control of Distillation Towers
2/20
Distillation separates mixture on the basis of a
difference in composition between liquid andvapour formed from the liquid.
In the process industry, distillation is widelyused to isolate and purify volatile materials.
-
7/28/2019 Instrumentation and Control of Distillation Towers
3/20
Distillation Equipment
FeedPump
Preheat
er
Column
The tower or columnhas two purposes:1) It separates the
feed into vapourportion whichascends the
column, and intoliquid portion whichdescends thecolumn.
2) Achieves intimatemixing between the
two counter currentflowing phases. Purpose of mixing is
to get an effectivetransfer of the morevolatile componentsinto the ascending
vapour and acorresponding
-
7/28/2019 Instrumentation and Control of Distillation Towers
4/20
Distillation Equipment
If the feed is liquid, thetemperature at which itfirst starts to boil iscalled the bubblepoint.
If the feed is vapour,the temperature atwhich it first starts tocondense is called thedew point.
The column is
operated in atemperature rangethat usuallyintermediate to the twoextremes of dew pointand bubble point.
For effectiveseparation of the feed,
FeedPump
Preheat
er
Column
-
7/28/2019 Instrumentation and Control of Distillation Towers
5/20
Distillation Equipment
The overhead vapourleaving the column issent to a cooler, andcollected as a liquid inreceiver. A part is withdrawn as
product or distillate. Remaining part is
returned to thecolumn as reflux.
The bottom liquid
leaving the column isheated in a reboiler. Part of the liquid is
withdrawn as bottomproduct or residue.
Remaining part is
vaporized andinjected into the
FeedPump
Preheat
er
Column
Reflux
Condens
er
Accumulator
Distillate
Reflux
Pump
Reboiler
Boil-up
Residue
This interaction
between the counter
flow of vapour and
liquid phase gives the
perfect separation of
volatile components
from liquid to vapour.
-
7/28/2019 Instrumentation and Control of Distillation Towers
6/20
Variables to Control
Independent variables are:
Column pressure.
Feed flow rate.
Feed composition. Feed temperature.
Reboiler (boil-up) control.
Reflux control.
Other variables are:
Level control.
Depends on the performance of independent variable
controllers.
-
7/28/2019 Instrumentation and Control of Distillation Towers
7/20
Most distillation control systems are based
upon maintaining the column pressure at someconstant value.
Variation of pressure will upset the control system bychanging the equilibrium condition of the material.
The set-point of pressure is the compromise
between two extremes. The pressure must be high enough to cause condensation of
the overhead vapour by heat exchange with the coolingmedium usually water.
The pressure must be low enough to cause vaporization ofthe bottom liquid by heat exchange with the heating mediumusually steam.
Pressure Control
-
7/28/2019 Instrumentation and Control of Distillation Towers
8/20
Column Pressure Control with
Liquid Distillate and Inerts
Presence of largepercentage of inert gascomplicates the pressurecontrol problem.
Uncondensable inert mustbe removed otherwise itwill accumulate andblanket off the condensingsurface.
LICLT
PIC
PR
C
PT
Vent
Column pressure can be
controlled by manipulating theheat transfer area inside the
condenser. If the pressure is less,
this control valve is closed by
using PRC so that liquid level
inside the pipe line just above the
CV increases, which will decrease
the heat transfer area, so that
condensation rate is decreased
(and the column pressure is
increased).
If the rate of condensate is large, level of liquid in
accumulator increases, which is sensed by the level
transmitter and opens the control valve, so that the
liquid level remains constant.
As the pressure passed a certain operating point (due
to the presence of large inert gas), PIC opens this
control valve so that inert gas with very small portion of
vapour is removed (because, the density of inert gas is
very small, and ascends the condenser tube, which isnot condensed).
This control valve is an ON-OFF control valve.
-
7/28/2019 Instrumentation and Control of Distillation Towers
9/20
Column Pressure Control with
Liquid Distillate and Negligible Inert Most common. Maintenance costs are low because the valve is on
the water line.
For properly designed condenser, PRC need onlyproportional controller, as narrow throttling range issufficient.
As the residence time of the water increases, it will
increase the time lag of the system. Wide proportional band is not sufficient.
Need automatic reset to compensate for the load changes.
Control the condensate which will maintain the water rateconstant.
Keep the condenser 10 to 15ft below accumulator.
LICLT
PR
C
PT
Water
Here the heat transfer area is constant. Column
pressure is regulated by manipulating the rate ofcooling medium.
-
7/28/2019 Instrumentation and Control of Distillation Towers
10/20
Column Pressure Control with
Vapour Distillate and Inert
System pressure will
quickly respond to
changes in flow of
cooling medium. Condenser will
condense only enough
condensate to provide
the column with reflux.
Condenser require a
short residence time.
LIC
LT
PR
CPT
Water
-
7/28/2019 Instrumentation and Control of Distillation Towers
11/20
Vacuum System
For some liquid mixtures, the temperature
required to vaporize the feed would need to be
so high that decomposition would result.
To avoid this, it is necessary to operate thepressure below atmosphere.
-
7/28/2019 Instrumentation and Control of Distillation Towers
12/20
Column Pressure Control with
Liquid Distillate and Inerts
Control valve of airstream is opened by PT,so that air pressurebecomes greater thanthe accumulatorpressure.
Air stream is flowedthrough the ejector.
Column pressureincreases
LICLT
PIC
PR
C
PT
Air
Stea
m
Click here to know the
effect if column pressure
P1 is too small (or morevacuum).
Click here to know the
effect if column pressure
P1 is close to theatmospheric pressure
Control valve of airstream is closed byPT, so that airpressure becomeszero.
Column vapour isflowed through theejector.
Column pressuredecreases.
Measurement of accumulator
pressure (which is almost
equal to column pressure)introduces less time lag than
the direct measurement of
column pressure
-
7/28/2019 Instrumentation and Control of Distillation Towers
13/20
Only six variables need to be fixedinorder to predetermine all conditions ina distillation column. The column pressure can be the first controlled variable.
If fixed feed conditions are assumed,
three more independent variables aredetermined. Feed rate.
Feed composition.
Feed temperature.
Feed Control
-
7/28/2019 Instrumentation and Control of Distillation Towers
14/20
Feed Flow Rate
FRC FT
By holding the flow rate
constant, best means of
operation stability is
obtained.Narrow proportional band
is used.
Feed
-
7/28/2019 Instrumentation and Control of Distillation Towers
15/20
Feed Composition
Feed composition have greater influence on
operation.
Feed composition is seldom subject to
adjustment.Changes are made elsewhere.
-
7/28/2019 Instrumentation and Control of Distillation Towers
16/20
Feed Temperature
FRC FT
For efficient separation, itusually is desirable to have thefeed at its bubble point when itenters the column. Unless the feed comes directly
from some preceding distillationstep, an outside source of heatis required.
Constant temperature feeddoes not necessarily meanconstant feed quality.
If feed composition varies, itsbubble point also varies.
Set the temperature control at apoint which is equivalent to thebubble point of heaviest feed.
TT
TR
C
Steam
To Column
SP
Feed
-
7/28/2019 Instrumentation and Control of Distillation Towers
17/20
Reboiler (Boil-up) Control
Boil-up rate iscontrolled bysetting the flow ofheat to the reboiler.
The amount oflighter boilingmaterial in thebottom product isdetermined by the
setpoint. Setting more steam
will driven largeamount of lightermaterial as boil-up.
LICLT
FR
C FT
Steam
-
7/28/2019 Instrumentation and Control of Distillation Towers
18/20
Reflux Control
LICLT
PR
C
PT
Since distillation achievesseparation of materials bycounter currentlycontacting vapour andliquid, reflux furnishes thecontinuous supply of liquid
to the column. Level control shown is not
an independent variablebecause, it reflux thedifference between whatcomes into accumulator and
what goes as reflux.
PT
PR
C
FR
C FT
-
7/28/2019 Instrumentation and Control of Distillation Towers
19/20
Variable Column Feed
Any inadvertent changeswhich occurs in the firstcolumn would be reflectedin the quantity andcomposition of the feed to
the second column. Liquid level controllers can
have a wide proportionalband so that changes inthe reboiler / accumulator
level can swing over a widerange without drasticallyupsetting the flow ofproducts. Second column will receive
varying flow rate, whichmust be avoided.
LR
CLT
FR
CFT
-
7/28/2019 Instrumentation and Control of Distillation Towers
20/20
Thank You